High frequency selective system and method



Oct. 3, 1944. R. s. BALDWIN 2,359,504

' HIGH FREQUENCY SELECTIVE SYSTEM AND mmnon Filed Au 10, 1945- r47 i 2m: 7- 41 a sgnal Shengfh INVENTOR.

Roberl' s. Ba/JW/n A TTORNE Y teristics of my invention;

Patented Oct. 3, 1944 2,359,504 HIGH FREQUENCY SELECTIVE SYSTEM AND ME'rnon 7 Robert S. Baldwin, Burlingame, Calif Application August 10, 1943, Serial No. 498,136

' 4 Claims. (Cl. 178-44) My invention relates to radio tube circuitshaving high frequency-selective characteristics.

Among the objects of my invention are to provide a, novel and improved system having exceptionally high frequency selective characteristics;

to provide a novel and improved system which 'is' capable of sharpening the frequency selective characteristics of a tube circuit; to provide a noveland improved system having exceptionally high frequency selective characteristics and which is capable of adjustment to any one of a wide range of frequencies.

Additional objects of my invention will be brought out in the following description of the same taken in connection with the accompanying drawing wherein-- Figure 1 is a system of fundamental design representing a preferred embodiment of my invention;

Figure 2 comprises curves illustrating the manner in which the system of Figure 1 functions to produce the high frequency selective charac- Figure 3 comprises curves illustrating the selectivity of a parallel resonant circuit as compared to the selectivity obtained whensuch circuit is embodied in the system of my invention, such as illustrated in Figure 1;

Figure 4 is a system representing a modified form of my invention;

Figure 5 comprises curves illustrating the manner of operation of the system of Figure 4;

Figure 6 is a system representing a further embodiment of my invention.

Ina preferred embodiment of my inventionv as depicted in Figure l, I provide an electron discharge device I which, reduced to its lowest terms, may be one of the three-element type comprising cathode 9, grid II and anode l3 electrodes. Between the grid and cathode electrodes, I provide a conventional input circuit which may take the form of a grid leak to the ends of which are connected signal input terminals 11. Such leak ,may be connected'directly or through ground to the cathode 9, or, as is often the case, the oathode lead may include a self-biasing resistor-condenser combination l9.

- The signal output circuit connected'between the cathode and anode, of course, includes the resistor-condenser combination l9, and, in addition, involvesa resistance 2| in series with a tunable circuit 23.

from the signal'output circuit by ablocking condenser 2I.

A connection 29 from the upper end of the output circuit resistor 2| to the grid if of the electron discharge device I embodies a resistor 3|, the connection having such phase as to degenerate the signal appearing in the input circuit of the electron discharge device i.

The tunable circuit 23 preferably includes a parallel arrangement of an inductor 33 and a condenser 35, either or both of which may be, of the variable type to permit of adjustment of the frequencyof the tunable circuit to any one of a wide range of values.

This tunable circuit is connected as a part of the input circuit of a second electron discharge device 31 which may also be of the three-electrode type, having cathode 39, grid 4| and anode 43 electrodes. As with the first electron discharge device 1, the cathode lead of the second device may include a self-biasing resistance-capacity combination 45 common to both the input and signal output circuit.

The signal output circuit minals 41 for connection to the load and is isolated from the direct current voltage supply circuit by a blocking condenser 49 as in the case of the firstelectron discharge device 'I."

A connection 5i from the upper load terminal to the grid ll of the first electron discharge device includes a resistor 53. This connection, however, unlike the corresponding connection 29 from the siganl output circuit of the first device, is so phased as to be regenerative in nature instead of degenerative.

The presence of the tuned circuit 23 in the system will determine the frequency to which the system will respond most efllciently but will not necessarily render the system highly selective as to frequency. The frequency selective characteristic of a tuned circuit varies .inversely with the such a circuit will always pass signals at fre- As is conventional'practlce, plate voltage for the device may be supplied from a voltage source (not shown) through a' resistor 25 and is isolated quencies adjacent the resonant frequency to an extent determined by the amount of resistance inthe circuit. v

, The reduction of the direct current resistance of a tuned circuit is particularly a problem in connection with circuits designed for operation at audio frequencies, in as much as the windings supplying the inductance for such circuits, require large numbers of turns. For this reason myinvention findsits greatest need in circuits designed for operation at the lower order of freincludes the load terfrequency characteristics are required.

Referring back to Figure 1, the system disclosed therein, in order to function in accordance with the teachings of my invention, must be so designed and adjusted as to bring about certain cooperative relationships between the various portions thereof as will be explained.

In analyzing the decay characteristics of any tuned circuit, it will be noted that such tuned circuit exhibits sharper frequency selective characteristics as the amplitude of the decaying wave diminishes, which means of course, that a par awide frequency range and of sufiicient magnitude to reduce the input signal voltage to an exceptionally low level, approaching zero value.

This I am able to accomplish by making the quencies, though it also has decided advantages in circuits at higher frequencies where sharper signal has been regenerated almost to its. origi- ,nalvalue, but at the desired frequency only.

In Figure 3, I have attempted to show by comparative curves, thefrequency sharpening effect of my system as compared to the normal frequency selective characteristics of a parallel resonant circuit. Referring to this figure, the dotted curve 6| represents the frequency response curve of such parallel resonant circuit, while the solid line curve 53 illustrates the sharpening "effect of my system on a signal when such parallel resonant circuit is embodied in the system.

In Figure 4, I have illustrated my invention as applied to a system designed to reverse the results of the system of Figure 1, and suppress a signal only at a particular frequency rather than suppressing it at all frequencies except at a desired frequency. The underlying principles of my invention however remain the same.

To accomplish such reversal of results, necessitates but a minor change in the-system of Figure 1, such change being illustrated in Figure 4 signal output circuit resistance 2| so high compared to the impedance of the tuned circuit 23, that the signal output circuit is predominantly resistive in character, and consequently the frequency response characteristics of the degenerative circuit will be 'very bread. The magnitude of such degenerating energy may be adjusted in large measure by the resistor 3| in the degenerator circuit.

The regenerative circuit of the second electron discharge device, on the other hand, is adjusted to feed back frequency selected energy of a magnitude sufficient to bring the suppressed input signal at resonant frequency up to a value approaching its former value or to a value at which the regeneration almost equals but does not exceed the value of the degeneration. In this range of values, the sum of the two feedbacks is negative with resulting stability of operation.

In as much as the second electron discharge device receives its exciting voltage from across the tuned circuit 23 operating at low voltage, the regenerator circuit 5| will be highly selective as to frequency, and consequently its regenerative effect on the suppressed input signal to the first electron discharge device I will predominantly occur at a particular frequency, leaving the signal suppressed as to the adjacent frequencies.

As a result of the cooperative effects of the features just described, I, in effect, strip the input signal of all its frequency components except the frequency component at which I desire to amplify the signal. Accordingly, the system exhibits exceptionally high frequency-selective characteristics.

Explaining the operation of my system with the aid of the curves of Figure 2, the upper horizontal dashed line 55 represents the original signal voltage which, in the absence of my invention, would be amplified, and indicates it to cirtend over a wide frequency range. The lower dashed line 51 represents the signal voltage as suppressed to a near zero level by the degenerative effect of the degenerator circuit 29.

to which reference will be made.

In this system, the signal output circuit of the first electron discharge device 1 includes a resistance B5 of relatively low value in series with the resistance 2| of high value, and across the resistance of low value, I connect a series tuned circuit 23'. The grid of the second electron discharge device is connected to the upper end'of this tuned circuit 23.

The resistance is preferably adjusted to a value about two or three times the resonant impedance of the tuned circuit. Since such resonant impedance is low in a series tuned circuit, the value of the resistance 65 will also be low.

The presence of resistance 65 has the effect of cutting off the reactance curve in such a way that the 'impcdancegof the tuned circuit 23 is effective only in the immediate vicinity of resonance. At frequencies further from resonance, resistance 65 is the controlling factor and the signal voltage is maintained in phase for use in i the regenerator circuit 5|.

Under these conditions, the signal impressed upon the grid 4| of the second electron discharge device 31 will be the reverse of that in Figure 1 and will embody essentially all of its frequency components except the component at substantially the frequency of the tuned circuit. Consequently the regenerator circuit 5| will feed backv energy at all frequencies embodied in the original signal except the frequencies in the immediate vicinityof the resonance at which the signal is suppressed by the tuned circuit.

By referring to Figure 5, it will be appreciated therefore, that the regenerator circuit 5| will regenerate the suppressed signal at the input of ,the first electron discharge device 1, to almost actistics of the tuned circuit In Figure 6, I have illustrated a modified form of my invention utilizing multi-grid electron discharge devices, and wherein the operating effects of my invention occur in a side circuit, and modify the frequency characteristics of the input signal through a separate control grid in the electron discharge device to which the input signal is applied.

More specifically, andwith reference to Figure 6, I provide. a multi-grid electron discharge device, 69 to the control grid ll of whichythe input signal is applied, as in the previously described systems. The signal output circuit of this electron discharge device may be of any conventional type but is shown as being of the resistance coupled type.

In the side circuit, I provide a multi-grid electron discharge device 13 and impress the input signal upon the control grid I of this device, preferably through a variable tap 11 on the input resistor 19 to this device, and connect the anode 8| of this electron discharge device to a direct current power supply in the same manner as in the previous systems. The signal output circuit from this electron discharge device will correspond with that of the system of Figure 1.

The tuned circuit 23 is connected in the input circuit of a three electrode electron discharge device 83 'whose signal output circuit terminals 85 are connected across a load resistor 81. The

cathodelead of this three electrode electron dis-- charge device includes a resistor 89 in common with signal input and signal output circuits of this device.

The degenerator circuit 29 of this system is connected between the upper end of the signal output circuit resistor 2i and a separate control grid 9| in the device, which grid is tied to ground by a leak 93 of high resistance.

The regenerator circuit 5| of this system is likewise connected to the same grid from the upper end of the load resistance 81 of the three electrode electron discharge device.

The suppressionand regenerative eflects'upon the input signal take place in this side circuit efiect be desired from the regenerator circuit as,

takes place in the system of Figure 4.

My invention may be employed in any system where higher frequency selective characteristics are desired than can be obtained through the reliance upon a tuned circuit only, and while I have disclosed but a few embodiments of my invention, it should be apparent that the embodiments disclosed and described by me are susdeptible to change and variation without departing from the underlying principles which characterize my invention, and Iaccordingly do not desire to be limited in my protection to the specific details of the systems disclosed and described by me except as may be necessitated by the appended claims.

I claim: y

l. A sharp frequency selective circuit comprising an electron discharge device having an input circuit for impressing a signal thereon and an output circuit, said output circuit being substantially resistive, a negative feed back circuit from the output circuit of said discharge device to the input circuit thereof for degenerating the signal voltage developed in said resistive output circuit, means associated with said output circuit for selecting therefrom a particular frequency comtron discharge device, said regenerative circuit 1 having a regenerative value sufficient to increase said degenerated signal to substantially its original value.

2. A sharp frequency selective circuit comprising an electron discharge device having an input circuit for impressing a signal thereon and an output circuit, said outputcircuit including a resistance and a frequency selective circuit, said frequency selective circuit being of low impedance compared with said resistance, a negative feed back circuit from the output circuit of said discharge device to the input circuit thereof for degenerating the signal voltage developed primarily across said resistance, a second electron discharge device having an input circuit and an output circuit, said input circuit including said u aforementioned frequency selective circuit, and a regenerative circuit from the output circuit of said second device to the input circuit of said first electron discharge device, said regenerative circuit having a regenerative value suflicient to increase said degenerated signal to substantially its original value.

3. A sharp frequency selective circuit comprising an electron discharge device having an input circuit for impressing a signal thereon and an output circuit, said output circuit including a resistance and aparallel resonant circuit, said parallel resonant. circuit being of low impedance compared with said resistance, a negative feed back circuit from the output circuit of said discharge device to the input circuit thereof for degenerating the signal voltage developed primarily across said resistance, said negative feed back circuit having a degenerating value sufllcient to almost nullify said input signal, a second electron discharge device having an input circuit and an output circuit, said input circuit including said aforementioned parallel resonant circuit, and a regenerative circuit from the output circuit of said second device to the input circuit of said first electron discharge device, said regenerative circuit having a regenerative value suflicient to increase said degenerated signal to almost its initial value.

4. A sharp frequency selective circuit comprising an electron discharge device having an input circuit for impressing a signal thereon and an 'output circuit, said output circuit including a resistance and a tuned circuit, said tuned circuit being of low impedance compared with said resistance, a negative feed back circuit from the output circuit of said discharge device to the input circuit thereof for degenerating the signal voltage developed primarily across said resistance, said negative' feed back circuit having a' degenerating value'sufllcient to almost nullify said input signal, a second electron discharge device having an input circuit and an output circuit, said input circuit including said aforementioned tuned circuit, and a regenerative circuit from the output circuit of said second electron discharge device to the input circuit of said first electron discharge device, said regenerative circuit having a regenerative value sumcient to effectively increase said degenerated signal'to a value approaching its original value.

' ROBERT B. BALDWIN. 

